A number of people suffer from thromboembolic diseases which usually occur in association with conditions such as atherosclerosis, trauma, cancer and premature separation of placenta. Therefore, an effective preventive measure is of utmost importance. The factors responsible for the onset of thromboembolic diseases are at least several, but one of the most important is tissue injury including vascular injury. When there is a tissue injury, one might reasonably expect that tissue juice including tissue thromboplastin might enter the blood circulation. Thus, tissue thromboplastin might be regarded as being directly responsible for at least some forms of thromboembolic diseases. We therefore propose to purify tissue thromboplastins from various tissues and to develop a sensitive method for the measurement of circulating tissue thromboplastins. BIBLIOGRAPHIC REFERENCES: Takeda, Y., Gonmori, H., Parkhill, T.R. and Kobayashi, N. Thrombosis and metabolic studies of fibrinogen, prothrombin, plasminogen and tissue thromboplastins, Plasma Protein Turnover, Ed., R. Bianci, G. Mariani, and A.S. McFarlane, MacMillan, London, Page 277, 1976. Kobayashi, N. and Takeda, Y. Effects of estradiol on antithrombin III metabolism in the dog. Fed. Proc. 35: 756, 1976.